texts: How accurate is Stellarium at depicting the Earth, from the Moon, approaching a solar eclipse (from Lunar perspective)?

texts:
This is how Stellarium depicts the positions of the Earth and Sun 24 hours before the solar eclipse (from Lunar perspective) on Jan. 13, 2131

A close up of the Earth on that day (Jan 12) shows no indication of any crescent Earth visible.

On Jan. 11, there is the barest of indications of a crescent Earth visible.
On Jan. 10, more of a crescent Earth is depicted.

Is there anyway of telling the accuracies of these depictions? Is there reason to think the human eye could see something of the Earth or its atmosphere on January 12, or would it appear entirely black?


texts: When a star runs out of  hydrogen, it either goes through supernova or forms white dwarf and planetary nebula. So there should not be much hydrogen left to form a new star in the nebula. So how new stars are formed from these nebulas?


texts: Can we measure comets in a similar way to exoplanets?

texts:
A 2km comet at 40AU's can dim a sun sized star at 100 light years by 3%, and a 10km comet can cause an eclipse (is that right?) So would 2,3,10 km objects be easy to measure via star dimming if we had enough telescopic data from space?

Can we measure lines of stars that are eclipsed/dimmed in sequence by transiting comets?

Given 1000 frames of a small patch of space, where all the images are recorded into a 3D image stack (a 3D array, a voxel) X,Y and Time as dimensions, then an Intel processor can compare 20 million voxels per second of that collection of images to search for vector information of aligned patterns of star dimming, i.e. searching for vectors of comets.

If that process is repeated for many patches of space, perhaps we can detect many comets, and I don't know the real maths involved, if a satellite of that design would realistically only find 1 comet, or 5000?

It's for a theoretical comet apprehension system which uses 2/3 identical satellites  each comprised 100Tb of SSD and a 20Mflops processors to search for vector information in the images.

Some kind of fairy idea going around in my head, because of Oumuamua and because I work on voxels and I know that image collections from space can be stacked into 3D arrays and be searched for vector patterns at 2million voxels per second on a 200W PC, an idea that I am confusedly astrally-projecting as a basis to find comets.

edit- A quantitative answer would be cool, say if the voids between stars on the milky way arm are 8 times the width of the comet, the comet would on average hit 2 stars every 8x8 frames.


texts: Where is the flaw in this hypothetical concept of escaping event horizon using another's blackhole flyby?

texts:
imagine two black holes approaching each other in a way that they will perform just very close flyby. You have a spaceship just below the event horizon of one black hole in line between centers of both black holes all the time.
When the black holes are closest to each other you should find yourself outside of the event horizon for a moment as the influence of one black hole should cancel out some part of the influence of the another black hole ( in similar fashion as the influences cancel out in the exact middle of a planet ); therefore, enabling you to escape. However, current physics says that nothing can leave the event horizon. Where is the flaw in this scenario?


texts: Understanding the angular peaks in the CMB power spectrum

texts:
I have read up quite a bit on the CMB power spectrum, but one feature evades my understanding. The first peak is often referred to as the "first compression peak", meaning that the plasma had time to compress exactly once. The way I imagine this is that we have a central overdensity, and then a ring of plasma around it which moves closer, i.e. the ring is being compressed. In the same vein I think of the second peak as corresponding to the ring being compressed and then rarefacted. But I don't see how this fits in with the power spectrum: the first peak corresponds to a larger separation between the center and the ring, while the second peak corresponds to a smaller separation. With the way I have described it, I believe it should be the other way around: compression should correspond to smaller scales, while rarefaction should correspond to larger scales.
What am I misunderstanding about this?


texts: I read an article on a group of stars orbiting the supermassive black hole Sagittarius A. Did these stars get caught by the black hole's gravity like planets capture moons, or were they born from the remnants of the star that went supernova and turned into black hole?


texts: How do they make International space station to orbit the earth beyond Earth's gravity acting on it? We all know that ISS is rotating at an altitude of just 350km away. How could the ISS escape Earth's gravitational pull?


texts: Are there ways to estimate size of the "whole universe"?

texts:
Words escape me, but by "whole universe" (I think) I mean everything that's spatially connected to the observable universe in a conventional sense. If there is a better term for it, please let me know!

I was surprised to find out that theories and measurements about the big bang seem to mostly relate to the size of the observable universe via expansion, and if I understand correctly don't say much about the size of the whole universe.

Have I got this right? Are there any ways at all to try to estimate size of the whole universe? Does the term even mean anything?

See @RobJeffries' excellent answer to When will the number of stars be a maximum? and @Acccumulation's comment for background on this question.


texts: I am new to astronomy. Jupiter is now visible to the naked eye but I have difficulties in locating it with my (new Newtonian reflector) telescope. The lowest magnification I have is 36X, which I guess is part of the problem(?). Any tips for the beginner..? I feel that if I knew any star constellations, it would help in locating objects.


texts: Could an asteroid knock the moon out of its orbit?

texts:
As describe in this CNN article, an asteroid is going to pass near the Earth on Halloween.

The article says that

  As it misses Earth by about 300,000 miles (slightly farther away than the moon), the asteroid, named 2015 TB145, will be visible to those with good telescopes

and that

  It's a good thing it will miss, though. The asteroid is estimated to be 300 to 600 meters wide and traveling at 78,000 mph.

If, hypothetically, this asteroid actually hit the moon, could it knock it out of orbit? How about another, contrived asteroid with whatever properties are needed?

To be clear, when I say "out of its orbit" I'm including both the orbit decaying and the moon crashing into Earth, and the moon being knocked out into space away from the Earth's influence. Either would do.


texts: Telescope buying guide for a beginner in India

texts: I am located in the southern part of India and am looking forward to buy a telescope to gaze up to look ay farther planets, moons in our solar system and take pictures. How should I go about getting a telescope as I know nothing about telescope and things I need to know?


texts: How gravity behaves for an Earth body and for a celestial body?

texts: Is there any difference in the gravity of the Earth for an Earth body or a celestial body? For example, will gravity be same for an Earth body, compared to the Moon?


texts: What is a good way to start learning Astrophysics

texts: I am a junior in high school and throughout my life I have always had a fascination with space. I use to watch "Space's Deepest secrets" and stuff like that but after a while I stopped learning much and became bored. Then I took an astronomy class in sophomore year which was fun until it ended. After that there wasn't much for me to do, but now I want to go from the beginning to the end. I want to learn and everything I can all over again, but this time I want to go farther. This is where my problem comes from, I have absolutely no idea where to start and would really appreciate any help that could be given.


texts: Can Gravitational astronomy look beyond the CMB?

texts: In another question dealing with the earliest explored phenomenon in the Universe the observing limit of the CMB was mentioned.  Prior to CMB the Universe was opaque to photons, I believe.  I'm wondering if conditions before "first light" would have allowed sufficient matter clumping to produce currently detectable, or potentially future detectable, gravitational signatures.  The irregularities in the CMB do indicate clumping of some sort, do they not?  I'm wondering if LIGO and it's successors will have a look back time farther than photon-based astronomy.


texts: Early universe's first starlight (indirectly) detected for the first time, could it have been detectable with 1970's technology?

texts:
The recent  paper in Nature An absorption profile centred at 78 megahertz in the sky-averaged spectrum seems (to me at least) to be an incredibly simple and elegant experiment with far-reaching implications.
While the system has been implemented very carefully, it is essentially a spectrally resolved radiometer at radio frequencies below 200 MHz, measuring the sky-averaged power spectrum. And while many of radiometers in space and on Earth measuring the Cosmic Microwave Background are fairly broad band, measure in the 10's to 100's of GHz range, and intended for very precise absolute intensity measurements of a smooth Plank distribution, the system described here measures between only 50 and 200 MHz, and with a spectral resolution of only 6.1 kHz!
The small set of carefully designed and optimized, antenna(s) and receiver(s) are simple and straightforward; a broadband dipole-type antenna above a large ground plane in a radio-quiet zone, some low noise electronics and what non-radio astronomers might call a very large bandwidth Software-defined Radio or SDRs.
And yet this small, simple receiver, sitting quietly on the ground in Australia's Outback has produced observations are amazing! They've detected direct evidence of the first stars producing the first starlight in our early universe only about 200 million years after the Big Bang!

Abstract:
After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line. This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz1. Here we report the detection of a flattened absorption profile in the sky-averaged radio spectrum, which is centred at a frequency of 78 megahertz and has a best-fitting full-width at half-maximum of 19 megahertz and an amplitude of 0.5 kelvin. The profile is largely consistent with expectations for the 21-centimetre signal induced by early stars; however, the best-fitting amplitude of the profile is more than a factor of two greater than the largest predictions2. This discrepancy suggests that either the primordial gas was much colder than expected or the background radiation temperature was hotter than expected. Astrophysical phenomena (such as radiation from stars and stellar remnants) are unlikely to account for this discrepancy; of the proposed extensions to the standard model of cosmology and particle physics, only cooling of the gas as a result of interactions between dark matter and baryons seems to explain the observed amplitude3. The low-frequency edge of the observed profile indicates that stars existed and had produced a background of Lyman-α photons by 180 million years after the Big Bang. The high-frequency edge indicates that the gas was heated to above the radiation temperature less than 100 million years later.

I don't see anything about this data that strikes me as unmeasurable with even say 1970's technology if done carefully. The signal is huge, a distinct, broad (about 19 MHz centered at about 87 MHz), several percent (almost 0.6 K) deep dip in an otherwise Rayleigh-Jeans regime blackbody distribution. In 20:20 hindsight, it seems to me this could have been detected a long time ago.
Question: Am I missing something, or is this discovery remarkable not only for it's far reaching implication, but for its simplicity, elegance, and the fact that it could have been discovered quite a long time ago with '60's or '70's technology?

above: Figure 1 from the paper.
below: Figures 2 and then 1 from the paper's Extended data.


texts: Is a supernova powerful enough to destroy the other star in a binary system?

texts:
(This question was originally posted via World Building, but I was instructed to post it here)
https://worldbuilding.stackexchange.com/questions/109457/is-a-supernova-powerful-to-destroy-the-other-star-in-a-binary-system?noredirect=1#comment332775_109457

The question is basically the same:

Can a binary star system have one of the two stars go supernova in the first place, and if so:

The first scenario would be:

Star A of the binary swells to red giant status, and then is either
touching or consumes star B: what would happen?

Scenario 2:

Star A goes supernova, with maybe 1 AU of distance between it and
Star B.

What info Id like to know:

Can it destroy the other star via supernova?
What is the effective    range of the supernova, and can it destroy
stars a light year or more    away?


texts: What are the curved lines in this X-ray image of the sky?

texts:
This image was created using the NICER instrument on the ISS.

Source:

https://www.nasa.gov/feature/goddard/2019/nicer-s-night-moves-trace-the-x-ray-sky

What are the big curves between the X-ray sources?


texts: How strong is the gravitational stretch we experience from the edge of the universe?

texts:
How much gravitational effect do we experience (e.g. maybe -.00001 G or smaller) from the edge of the visible universe?  By edge of the visible universe, I am talking about the region of the cosmic microwave background and beyond.  In the past, the universe was very hot and dense, and we are seeing the light from this time period as the Cosmic Microwave Background.  It took slightly less than the age of the universe for that light to reach us, and it has become extremely redshifted with the expansion of space.  Since gravity apparently travels at the speed of light, we should be experiencing a gravitational effect from this shell around us as well.  As I understand, the universe was extremely dense back then, and it is around us in every direction, which would intensify the gravity greatly.  However, it is extremely distant, which would weaken the gravity greatly.  I don't know if redshift would affect gravity as it has the light from the cosmic microwave background, but if so, this would weaken the effect greatly as well.  With all this in mind, I am curious, what would be the effect of gravity from the edge of the universe on us?

Note: I would expect this to be a very small outward effect in all directions, something like -.00000001 g or something like that, but not 0.


texts: Why aren't globular clusters disk shaped

texts: I thought that solar systems and galaxies are disk-shaped because that is the most stable shape under gravitation. Globular clusters are very old, often times older than their host galaxies, so why haven't they flattened out?


texts: How fast would an object at cruising altitude(around 10,600 m) need to travel to stay on the opposite side of the Earth to the Moon?

texts: I am trying to figure out how fast an object will have to travel (at a height around 10,600 m) to never be in the presence of the moon. Meaning the object will always be on the opposite side of the moon to the Earth.


texts: Mathematically calculate if a Planet is in Retrograde

texts:
I am trying to find out if a planet is in an apparent retrograde motion with respect to the earth at any given point in time. Given the time in Julian days.

I already have the geocentric and heliocentric coordinates and velocities of the planet using the JPL ephemeris, but not sure of what mathematical formula to use to identify the retrograde motion.

I read this article "https://physics.stackexchange.com/questions/249493/mathematically-calculate-if-a-planet-is-in-retrograde" which had a similar question but I am not sure of:

Is the answer correct as it has not been marked as correct?
How to represent earth in the XY plane only when it has XYZ planes and why do we even need to do that? Can't we just use the heliocentric equatorial plane which I think is the same as the one used by NASA JPL ephemeris.


texts: May anyone tell me does it known if Enceladus has water and is that water salty?


texts: How to calculate the angle formed between 2 planets?

texts:
I am interested in calculating the angles formed between planets like Saturn opposite to Neptune - 180 degrees, PlanetX opposite/trine/square to PlanetY. How to find such degrees?

I use Stellarium software to find positions of planets. Here I am adding positions of Saturn, Jupiter and Sun.

Stellarium shows information about selected objects, can it be used to calculate angle formed, for example, angle formed between Saturn and Jupiter taking Sun as reference point?


texts: V471 Tauri's circumbinary brown dwarf non-observation; Applegate, or over-restrictive assumptions?

texts:
tl;dr Has the brown dwarf observation been disproven?

I have just started reading about the interesting object V471 Tauri. The first two sentences of the introduction to The V471 Tauri System: A Multi-datatype Probe Vaccaro et al. (2015):

  V471 Tau, a white dwarf-red dwarf eclipsing binary (EB) in the Hyades with orbit period $0^d.52118$, is primarily known for its unique historical role as a stimulus to common envelope evolution theory (Chau et al. 1974; Refsdal 1974; Sparks & Stecher 1974; Ostriker 1976; Paczynski 1976; Alexander et al. 1976; Taam et al. 1978). Other properties include a likely brown dwarf companion to the EB, measured white dwarf spin, mass loss and exchange in a detached binary, differential rotation measured via magnetic spots, spot distributions, accurate white dwarf parameters, and photometric-spectroscopic distance measures that help to pin down the binary’s location within the Hyades.

It's the "likely brown dwarf companion" I'd like to ask about. The paper with the catchy title "The First Science Results from SPHERE: Disproving the Predicted Brown Dwarf around V471 Tau" Hardy et al. (2015) SPHERE is a new, advanced adaptive optics system at the Very Large Telescope (VLT). The image below (figure 3) is part of the argument, and the suggestion is that if there is no brown dwarf seen in the band between the two white circles, then the predicted brown dwarf does not exist.

This would be interesting, because an alternative explanation for the slow, periodic drift in the eclipse timing would have to be found. One possibility is the Applegate Mechanism which I don't understand, but may ask a separate question after this.

Back to Vaccaro 2015, section 9, titled "On the Reality of the Third Star" is over six pages of discussion of the underlying assumptions, and if I understand it correctly provides several possible ways in which a suitable brown dwarf could exist but not show up in the SPHERE image. In essence disproving the existence disproof.

I would like to know: Is my understanding of the current situation correct? Have there been more recent developments?

above: Left panel of Fig. 3 of Hardy et al. 2015: "Figure 3. H-band image of V471 Tau obtained on the SPHERE IRDIS instrument at the VLT. Left panel: Resulting image after angular differential imaging (ADI). The area in-between the white circles denotes the 5 sigma predicted position of the brown dwarf..."

above: The monster currently known as SPHERE from here.


texts: Do comets contain any significant amounts of iridium?

texts: I have Googled "iridium content of comets" and can't find anything that suggests comets contain iridium in any detectable quantities. Yet, whenever I read an article about the Chicxulub impactor, the article invariably says something to the effect that it may have been created by an asteroid or a comet. But Alvarez and company first offered the impact hypothesis for the K-T extinction by observing an iridium layer in global sedimentary deposits, which could only have come from an asteroid impact, since iridium is extremely rare in the Earth's crust. Later the Chicxulub structure was identified as the probable site of the asteroid impact. While a comet could have caused the Chicxulub crater, it could not have been responsible for the global iridium layer. So why do science writers offer the comet alternative? If Chicxulub was created by a comet, then another site must have been the source of the iridium layer.


texts: Is the measurement of distance and position of remote celestial bodies accurate?

texts:
Considering that light is affected by gravity, how accurate are measurements of distant stars and galaxies?
When light passes through objects with great mass, such as Jupiter size planets, stars, or black holes, shouldn't it change its course and result in a deviation of trajectory? Does the light we see with telescopes when observing the many celestial bodies in the sky pinpoint the real location of such bodies in the past, or was its past location divergent from the point in space indicated by the light reaching us today?


texts: Will I age slower in space or on other planets?

texts:
I came across few  videos which says Astronauts in ISS will have 0.07 seconds extra life.

So if life is possible on mars, than what is the time period of life for us humans? Is it the same as that of earth or different?

For example, if I spend 500 days on mars, that will be equal to spending around 515 days on earth. So is it that I'm gaining 15 days of life? I mean, can a human life span increase?

If a human average life span is 60-80 years on earth, on mars will the average life span of a human be 62-82 years of earth years? Or will it be 60-80 years of Mars years?

I don't know how much this makes sense but as a new learner just got a thought on space time.


texts: How big does the moon have to be to have the Earth tidally locked to it

texts: I would like a earth-moon system like Pluto-Charon (but with earth instead of Pluto) where both will be tidally locked to each other within something like a billion years. As the moon would probably lock to earth before the opposite how big the moon has to be?


texts: How can the universe be expanding faster than speed of light?

texts:
So the story goes like this:

A long time ago, 13.799±0.021 billion years to be exact, something happened. It was a big bang, loud explosion and universe came to existence. It grew and grew, and now we know that the observable universe is about 93 billion light years in diameter. So it means that

  objects that were once close ... are now up to around 45.7 billion
  light years away rather than up to 13.799 billion light years away ...

Hence my question is that everything is traveling faster than the speed of light?

If it is, then what does it mean in terms of special relativity?

I did read:

Is the universe expanding faster than speed of light?
How fast is the universe expanding?


texts: Is there a difference between the solar elevation angle and sun declination?

texts: I need to use the sunrise equation but one of the variables is the sun declination. On the other hand, I have the values for solar elevation angle that I need. Are they the same thing?


texts:
I understand that astronomers once thought that the material in the disc of a galaxy was moving around the galactic centre (where most of the mass was thought to be) in roughly circular orbits. The circular motions would be explained by the combination of inertia and gravitational acceleration towards the centre. This follows the Kepler/Newton model which describes/explains the orbits of planets around the Sun.  In a Kepler/Newton system the tangential or transverse velocity of a low-mass orbitting object depends on the radial distance of the high-mass object from the centre.  Objects further out must have lower velocities so that the weaker centrally-directed acceleration at that distance can keep the object moving in a roughly circular track.

In the centre of the galaxy the velocities are low and increase rapidly with distance away from the centre.  This is understandable as the mass of the central bulge of the galaxy  is not concentrated at the centre, it is spread out over a relatively large volume.

However it is now well-known that the measured velocities of visible material in the disks of discoidal galaxies do not vary as expected for a Kepler/Newton system.  At  a certain distance where the velocities would be expected to start decreasing (as per curve A) they either level out or continue increasing at a slow rate (as per curve B).

The current explanantion is that a large mass of Dark Matter extends throughout the galaxy in such a way as to produce non-Keplerian behavior.

QUESTION

But isn't there another possible explanantion of the non-Keplerian velocity profile namely that the ordinary detectable material (e.g. gas, dust, stars) in the disk is not completely gravitationally bound to the galactic centre and is actually gradually moving outwards along a spiral path?


texts: Is there a publicly available Gliese Star List

texts: Is there a publicly available list of the Gliese catalogue of nearby stars with their constellations? or a Gliese to Hipparcos List ?


texts: How did Kepler come to the potencies in his Third Law?

texts:
How did Kepler come to the conclusion,  that exactly the square of the period and the third potency of the great semi-axis of the ellipse is proportional?
Why is only square divided by cubic = constant?  Why do other dimensions not work? Kepler certainly did not simply try all potencies.
Isn't this a similar problem to Fermat's last theorem?


texts: Is there still a possibility that Makemake is not ellipsoidal but asteroid-shaped?

texts: In 2008, Makemake and Haumea have been named under the expectation that they would prove to be dwarf planets. Well, did they prove to be ones? Haumea obviously did since we know its shape which is a pretty weird ellipsoid. What about Makemake? Did we learn meanwhile whether Makemake is in hydrostatic equilibrium and/or whether it has an ellipsoidal shape? Or is there still the possibility that it's no dwarf planet according to the IAU's rules?


texts:
If the expansion of the universe (the metric) continues (and perhaps is accelerating), in a very large but finite time the expansion of the metric will clash with the effects of the strong nuclear force. What happens then? Would the energy required halt the expansion and if so what is the slowing effect of the "stretch" on the strong force today - too small to be measured?

Update: Thanks for the answers so far - I hadn't even considered the big rip scenario. I was merely thinking that as the metric expanded the strong force inside baryons would require unlimited resources of energy to be overcome - but I now understand that is wrong however it still looks to me like a potential perpetual motion machine - continually creating matter/anti-matter pairs. Perhaps that should be a separate question though...


texts: What is time period of sun revolving around the center of Milky Way?


texts: If we could hear black holes, how loud would they be? I know that this is basically the same question as the question "How loud would the Sun be?", except this time there is a black hole. But since the black hole sucks in stuff like a passenger jet engine sucks in air, about how many decibels would it be? Also, would it lethal to humans if we were 1 AU away and what would it do to our planet?


texts: What is the formula to predict lunar and solar eclipses accurately?

texts:
A number of ancient civilizations had devised methods to predict exact dates and times of such eclipses, marking them as important events.
Hence I assume the predictions were based on calculations, which should be quite easy to do now.
So what is the exact formula to predict the exact date and time(this is optional, but desirable) of lunar and solar eclipses?
Also how to calculate if the solar eclipse will be visible from a particular location or not?


texts:
Astronomic observations in geodesy.

It is well understood that sun and star positions may be expressed in Greenwich hour angle and declination. Or Right Ascension Declination. And may be published reckoned to Universal Time (UT), or TT.

An approximation of UT can be obtained by apply a correction (DUT) to UTC. That results in an expression of time described as UT1. DUT time offsets are published and broadcast (WWV shortwave radio) in 0.1" increments.

I'm looking for daily precise (better than 0.1") DUT corrections.

Current broadcast DUT correction is -0.2".

I have retrieved IERS Bulletin B, which has daily values of UT1-UTC in ms. Current value is -21.996 ms. Is it just me? Is -21.996 ms = -0.022"?

Or is there something in the IERS Bulletin B that I'm missing?


texts: Is the Big Bounce Theory valid? If so does that mean all this have happened before?

texts: IF the universe is just an infinite loop of events from expanding to contracting does that mean all this has happened before? The initial conditions for the next big bang would (or rather could) be the same as ours which would result everything happening in the exact same way. I could be way off but it's really strange thinking all of this has happened "before" like the time of arrow is somehow looped and we're all living the same life over and over again without even realizing it.


texts: How can the Sun block an X-ray telescope from observing a QPO galaxy "for several years"?

texts:
Phys.org's Black hole's heart still beating  says:

  X-ray satellite observations spotted the repeated beat after its signal had been blocked by our Sun for a number of years.

and links to the Open Access PNAS paper (Jin, Done and Ward 2020) Reobserving the NLS1 galaxy RE J1034+396 – I. The long-term, recurrent X-ray QPO with a high significance. (also arXiv)

I can't imagine any kind of satellite orbit (Lagrange-point associated, Earth orbit, Heliocentric) for which the direction of the Sun would prevent a suitable orientation "for a number of years" assuming that number is greater than 0.5.

Why was this so?

related in arXiv: Re-observing the NLS1 Galaxy RE J1034+396. I. the Long-term, Recurrent X-ray QPO with a High Significance


texts: What's the minimal argument I can give to reject The Geocentric Universe?

texts:
I'm new to Astrophysics, So pardon me if you find it silly to ask. I was just started reading about the basics of astrophysics. I read about the Greek Tradition, the concept of a geocentric universe. If I were asked in my childhood, to look at the sky and tell what do you think about the clockwork of the planet, I would have said that the Earth is at the center and everything moves around it as said by the greeks.
I read, How different peoples found difficulty explaining the motion through this Hypothesis. Though they were able to make some complicated models understand the wandering stars etc. And then Copernicus suggested a heliocentric model of planetary motion which is as believed, to make things simpler to make calculation simple. And So on...
It seems the things were based on more or less explaining the data by making an appropriate model and that's How science more or less works.
I want to know if there is some more basic argument that could be given to reject the Geocentric universe.
If not, Could there be some basic experiment that can be done to approve the Heliocentric Hypothesis and reject the other?

Edit: One more thing to add, Apart from the minimal argument, it would be great if the argument would remain valid at that time that is it should not contain any new information that wasn't known at that time.


texts: Blood Moon Identification in NASA reports

texts:
Often I look at NASA's Eclipse Web Site. More specifically, I often look at the Index to Five Millennium Catalog of Lunar Eclipse. How can I tell if an eclipse results in a "blood moon"? I know not all lunar eclipses result in a blood moon, but I do know that some do. So how can I know if one is from looking at it? A blood moon is the phenomenon whereby the moon in total eclipse appears reddish in color as it is illuminated by sunlight filtered and refracted by the earth's atmosphere. (Google)

Edit:

James K says this question may be a duplicate of another question. I want to know how to identify "blood moons" from looking at NASA's reports. The other question is more general. However the answer may be the same.


texts: How do solar winds affect the atmospheric composition and density of planets?

texts:
Here is a bit of a newbie question, if you please.

The sun is constantly ejecting small particles via solar winds that come into contact with planets and other objects in our solar system. Do these particles contribute to the planets's atmospheres? Or do they do more harm than good? (define: good = contribute).

Do solar winds contain any larger atoms than helium. For instance, Oxygen?

My intuition on this issue is mixed since we have Mars with a very thin atmosphere, but Venus with a very thick atmosphere, neither having a very strong magnetosphere to protect them from solar winds.


texts: Fraction of circumpolar stars to all stars visible in a location

texts:
We know that circumpolar stars are stars that don't rise or set. If we are in a location with $\phi = 35^o N$. What is the fraction of circumpolar stars to all stars visible in this location?

I know the declination of this stars is $\delta > 90 - 35 = 55$ But I don't know what is the degree of all stars visible in this location and is the fraction as simple as dividing this two angles or not?


texts:
I was using SkySafari with my phone, but the compass didn't quite work.
So as I knew where the cardinal points were, I think I have shot the Sagittarius star cloud, at least a part of it.

Can you guys tell me if I am wrong?

It was the 5th of July at 00:10 local time. I was at N 45.61°, E 7.35°.


texts: How does the length of the minor semi-axis of the moon's disc's inner terminator vary through the cycle?

texts:
The curve that separates the bright and dark parts of the moon's disc as viewed from the earth is called the inner lunar terminator, which is semi-elliptical in shape. We can model the disc as a unit circle centred on the origin, in which the inner terminator runs from (0,1) to (0,-1) and its major semi-axis always has length 1.

How does its minor semi-axis vary during the lunar cycle? Please assume that orbits are circular.

In other words, if the point halfway along the inner terminator is X, what is the equation for the motion of X back and forth along the line-segment from (-1,0) to (1,0)?

Note that the shape of the visible disc is not a lune, except in the two trivial cases.


texts: Why does a right-handed circular polarized wave get lagged when going through ionized plasma?

texts:
I have a question related to Faraday rotation. In Abigail Polin's Faraday rotation video I have seen that a linearly polarized wave can be decomposed in a left-handed circular wave and a right-handed circular wave and that when passing through ionized-magnetized plasma one of them gets lagged due to a difference on their refraction index.
Moreover, the video shows that the wave that gets lagged most is the right-hand circularly polarized.
Why is not the left-hand circularly polarized? What is the mathematical difference between their refraction indices?


texts: Critical density of the universe (no, not that one)?

texts: I seem to recall reading something that said if the density of the universe was off by as little as one atom per cubic meter, the universe, as we know it, would not exist; one less and there wouldn't have been enough gravity to form galaxies and stars, one more and there would have been too much matter. (I don't know (or remember) what would have happened in the case of too much matter.)If I Google the title of this question, all I get is stuff about the expansion of the universe, nothing about whether or not it would exist. I've tried other variations but can't seem to find anything.Did I grossly misinterpret this? If not, does anyone know of any articles that can confirm or refute this?


texts: Will I be able to see both Jupiter and Saturn at the same time in my Astromaster 114?

texts:
I have a Celestron Astromaster 114 Newtonian reflector telescope. The specs:

aperture: 114mm
focal length: 1000mm
focal ratio: f/8.77

I also have a t-ring so I can connect a Nikon camera to it.
Will I be able to see both Jupiter and Saturn at the same time thru this telescope? I understand from this article that they will come within a tenth of a degree of each other. The article says this is "fifth of the width of the full moon" which, based on my experience, suggests that I will indeed be able to see both, but I'm hardly any expert.
I'm considering borrowing a friend's film camera to try and snap a photo of both in one image, but would like to avoid the trouble (thanks, COVID!) if this won't happen.


texts: Was the "green star" event in NGC 3314 ever figured out or named?

texts:
from NGC 3314 Variable Object by By Bill Keel (University of Alabama, Tuscaloosa) and Lisa Frattare (Hubble Heritage Project, STScI). The object in question is circled.

Comments below this answer led to Dust in Galaxies which shows the image of the two visually overlapping galaxies together called NGC 3314 which says

In a March 2000 observation of the galaxies, a prominent green star-like object was seen in one of the arms. Astronomers theorized that it could have been a supernova, but the unique filtering properties of the foreground galaxy made it difficult to decide definitively.1
1Keel, Bill; Frattare, Lisa. "NGC 3314 Variable Object". heritage.stsci.edu. Retrieved 2015-11-05.

which explains the story, including:

Tracking down a stellar explosion
We first noticed a new object in NGC 3314 while discussing the details of color rendition for the Hubble Heritage release, incorporating data from both the 1999 and 2000 observations in all four filters. There was a prominent green starlike object in one of the spiral arms. "Prominent" is relatve here; the visual magnitude was 21.6. Green stars are not found in nature, because the spectral band we see as green is narrow compared to the amount of light stars put out in various wavelengths, so this was a signal of something quite unusual. This was quickly traced to an object appearing only in the March 2000 observations, so that combining data from some filters in which it was there and some in which it wasn't produced the giveaway color. The fact that we had blue-light images from both sets of data let us show that it had indeed newly appeared, and was not some object with a bizarre spectrum that vanished in some filters. In asking what kind of new object this was, we could quickly eliminate several possibilities:

and ends with

To get this observation on the record, and seek confirming or prediscovery observations, we reported it to the Central Bureau for Astronomical Telegrams, who included it in International Astronomical Union Circular 7388. To date we've gotten no reports of other observations, or of helpful non-detections in 1999 or 2000. So if you happened to take a deep image of NGC 3314 during this period...

Question(s): Rather than ask a half-dozen questions that likely won't be answerable individually, I'll lump them here. If there are answers to some but not others I'll accept them and edit the question to narrow it to what's answerable.

Were any more images found?
Was some part of a light curve established?
Was the object ever confirmed to be a supernova?
Does this "little green object" have a name or identifying designation of some kind beyond "NGC 3314 Variable Object"?
I can not understand the explanation for why it was thought to be green, is there a way to express it in simpler terms?

IAU circular 7388 says the following but doesn't give a name

POSSIBLE NOVA IN NGC 3314
W. C. Keel, University of Alabama; and L. M. Frattare, Space
Telescope Science Institute, on behalf of the Hubble Heritage Team,
report the detection of a possible nova in the overlapping galaxy
pair NGC 3314A and 3314B.  HST WFPC2 observations with the F450W,
F555W, and F675W filters on Mar. 10.47-10.57 UT show a new stellar
object that was not present on WFPC2 F450W (to B about 25.5) and
F814W images from 1999 Apr. 4.  Standard (approximate)
transformations to the UBV system give magnitudes B = 22.42, V =
21.64, R = 20.87.  The new object is located at R.A. =
10h37m12s.82, Decl. = -27o40'51".5 (equinox 2000.0), which is 1".2
west and 10".3 north of the optically brighter foreground nucleus
of NGC 3314A (itself at position end figures 12s.91, 41'01".8).


texts:
Question:
Approximately once in 5 years people on Earth can observe bright comets whose nuclei have radius about 1km. The orbits of such comets are close to parabolic. Assuming that these nuclei are uniformly filling the volume of spherical Oort cloud with radius 10000 AU, estimate the number of large comet nuclei and mass of Oort cloud.

I'm not sure how to even start this question because I don't get how the comets uniformly fill Oort cloud yet on Earth we can only observe comets once every 5 years (unless there are some things blocking our views of these comets?). Would appreciate any help.

P.S. Relativity should not be needed in this question...


texts: Why is the solar system often shown as a 2D plane?

texts:
Whenever I have learned about the solar system I always see the orbits displayed as a virtually flat plane.

Are all of the orbits in the solar system really like this? If so, why? It seems like a rather large coincidence for all of our planets + the asteroid belt to be on virtually the same plane.


texts: How often are new astronomical objects (variable stars, supernovae, comets, etc) discovered by amateurs? Where could one report new findings?


texts: How many minutes does a sundial change every day?

texts:
I'm planning to create a sundial in Ghana (10° 28.852', 0° 26.032') in July 2014.
I will stay there for about 3 weeks.
Do I need to adjust the numbers anywhere in these 3 weeks?
I know the shadow's path changes through the year, but how much does it change?
Is the change significant for a sundial?

(btw, is this the correct site to ask or should I use the astronomy site?)


texts: Convert dates to julian day number in astropy?

texts:
I'm using astropy and I need to convert dates like this:
2012.06.22_17:56:15_TAI
into julian day number. I tried:

from astropy.time import Time
hmitime = hminew['MidTime']
i=0
print(hmitime[i])
hmi0=Time(hmitime[i],scale='tai')
print(hmi0.jd)

where hmitime it's just the matrix with my dates. But python shows me this:

ValueError: Input values did not match any of the formats where the format keyword is optional [u'datetime', u'iso', u'isot', u'yday', u'fits', u'byear_str', u'jyear_str']

Does anyone know what is wrong? How can I convert my dates??
Thanks


texts: If we assume that there is nothing except the sun in the universe then how far the light can travel ??


texts: How is stellar mass of a galaxy obtained?

texts:
I believe there is a difference between the mass calculated using the orbital speed and the stellar mass of the galaxy. So how is the stellar mass calculated?

Thank you :)


texts:
I am trying to get data from Gaia. Especially for Cepheids or RR Lyrae. On Gaia websites it say they have released some of the data however I cannot find it. I am looking for two columns with time and magnitude(light curve).

Thanks for help


texts:
Although the Martian atmosphere is thin, there are many atmospheric phenomena that occur on Mars: storms, dust devils, (carbon dioxide) snow,... Some storms even stir up the atmosphere of the entire planet!
During Martian storms, is there any lightning and thunder, like what we see during Earth storms? What about in the past, when atmospheric conditions were different, has there ever been any lightning on Mars?


texts: How many nested stable (1 Mio years) orbits are theoretically possible?

texts:
If we look at our solar system we have the sun which has planet orbits and planets have moon orbits.

If we look at the possible size difference from hypergiants to small asteroids why shouldn't it be possible to nest orbits to an astounding degree: supergiant orbit a hypergiant, star orbits supergiant and so on ?

My first guess is that the size and distance must be so small in contrast to the primary that the gravitational gradient of the primary does not cause too much disturbance, but it should be possible.

Has anyone tried to find it out with celestial mechanics ? While it may be completely theoretical, it is IMHO a solid question and it would be nice to find out how deep celestial bodies can be nested and how it limits stability.

Some conditions:

The orbits should be (very probably) stable for one million years.
Hypergiants are burning fast, so I limit the stability for a relative short timeframe.
The barycenter of both satellite and its primary are inside the primary.
No double system.
The first primary should be a star, not a black hole or a neutron star.


texts: Is it at all possible for the sun to revolve around as many barycenters as we have planets in our solar system?

texts: Though it is understandable that the sun and the earth may be revolving around a barycenter, but, if so, not only the sun and Jupiter should also be revolving around some barycenter, the same should be true about the other planets as well? So it has to be true that the sun revolves around as many barycentres as the number of the planets we have in our solar system. I am quite confounded — how any object may at all revolve around multiple barycenters? And if an object can't revolve around more than one point, does it not nullify the theory that the sun and the earth revolve around a barycenter?


texts: What would the sky have looked like when galaxies first started to form?

texts:
If I stood on a theoretical earth at the time that galaxies were just starting to form, what would the sky have looked like?

Due to the smaller size of the universe would it have been densely packed with objects? If yes, what might I have seen?


texts:
I know this is a simplistic question, but I cannot find a straight answer...
Also, is it possible most or all neutron stars are magnetars and/or pulsars?  It's just hard to see all of their properties from our vantage point, etc.?


texts: If gaseous planets have solid core, can they theoretically be considered as rocky planets?

texts:
I read that gaseous planets such as Jupiter and Saturn, have a solid core deep inside. So, I wondered whats the difference between them and rocky planets apart from the fact that they have - what can supposedly be considered, a very thick atmosphere, and that their atmospheric pressure would probably be too high.

Is there any other difference which I am missing? I mean, if we go by definition, couldn't all their gaseous content, be considered as an atmosphere?


texts: How can pre-main sequence stars radiate more energy than main-sequence stars?

texts: How can a pre-main sequence star radiate more energy by gravitational contraction than a main-sequence star can by hydrogen fusion?


texts: Does the density in the core of a "higher mass" star stay generally constant while it evolves on the MS?

texts: We know the CNO cycle produces the majority of energy production in a "higher mass star" of approx. over 2 solar masses and the core is convective due to the large temperature gradient. My professor has noted that radius, luminosity, T_eff, and T_center all increase but rho_center (density) does not. I cannot find anything in my text that explains this, but I assume it is because convection works against gravity. If someone could clear this concept up (as to why rho_center) stays generally constant, I'd appreciate it. Conceptual explanations are helpful!


texts: How was the core temperature of the Sun estimated?

texts: It was estimated that the heat inside the core of the Sun inside around 15 000 000 °C - this value is extremely enormous. How did scientists estimate this value?


texts: Could a gaseous moon exist in the same way as a giant gas planet? All the moons in the solar system are rocky, or icy. Why shouldn't gas planets  have gas moons?


texts:
https://en.m.wikipedia.org/wiki/Three-body_problem

Have three or more celestial bodies rotating each other in a stable manner ever been observed, or it is only a theoretical problem?


texts: Mechanisms of binary/multiple star formation

texts: What are mechanisms of binary/multiple star formation in different mass ranges (low, intermediate and high stellar masses)?


texts: Is it true that the sun goes down to the west?

texts: I know the sun doesn't go down but we are moving and is it true that the sun goes down to the west? I've read on many webpages that this is not entirely true.


texts: How many meteorites have been found some place other than Earth? Where is an authoritative list maintained?

texts:
Backgound:
After seeing this tweet by astronomer Kacper Wierzchoś about our not-so-new but at least newly-discovered "mini-moon" I scrolled down and saw the tweeted image below.
The object is the known short-period comet 114P/Wiseman–Skiff. The article goes on to say:

Comet 114P/Wiseman–Skiff is believed to have been the parent body of the first meteor photographed from Mars

Context:
and that article says

As on Earth, when a meteor is large enough to actually impact with the surface (without burning up completely in the atmosphere), it becomes a meteorite. The first known meteorite discovered on Mars (and the third known meteorite found someplace other than Earth) was Heat Shield Rock. The first and the second ones were found on the moon by the Apollo missions.

This doesn't mention Egg Rock (as discussed in Who discovered “Egg Rock”? The Curiosity rover or people?) so we know that there must be at least four meteorites found some place other than Earth.
But I don't yet know how many there are in total
Question: How many meteorites have been found some place other than Earth? Where is an authoritative tally or listing actively maintained?
from EarthSky, credit NASA/JPL/ASU, captions "October 30, 2016 image via Curiosity rover on Mars" (click for larger size):


texts: Distribution of spectral types of "giant" stars

texts:
I'm looking for the distribution of spectral types in the "giant" star category. This category is defined as basically B and O main sequence stars and any star that is larger than main sequences outside of it.

My first thought was that the M, K, G, F, and A type distribution were the same as they are in the main sequence but then I realized M has to be dropped, and then i'm not quite sure how K, G, F, and A types evolve and usually only Red Giants are mentioned and I couldn't find numbers for the giants so I'm stuck.

To be clear, I want to know the percentage of giant stars that are in each of the spectral types. So if there are 1000 stars in a given area and 100 giant stars in that areas I only want to know the percentage of each spectral type of the stars in that 100.


texts: Isn't the date of the Big Bang a bit bogus?

texts: If I understand correctly, the date of the Big Bang is an extrapolation of acceleration of the universe's expansion through time based on the erroneous assumption that the universe is approximately 13 billion years old. The age of the universe, as far as I can tell is based on the speed of light and the fact that the furthest we could possibly see into the depths of the universe is approximately 13 billion light-years. Isn't that sort of a cat chasing his tail?  It seems that there is no valid reason for believing that the universe is really of a radius of ~13 billion years. That is just as far as we can see. If this is wrong and there is a valid reason for believing that the universe is ~ 13 billion years old, then that fact makes for a very interesting possibility.  That the expansion rate of the universe is equal to the speed of light, which leads to a very interesting suggestion that I pursued for quite awhile (until it became apparent that the suggested size of the universe is not what it seems), that the expansion rate of the universe is equal to the speed of light. Or, put another way, is it possible that the speed of light is based on the expansion rate of the universe?


texts: A quantitive estimate of the size above which celestial bodies must be near-spherical

texts:
Asteroids and smaller celestial objects tend to be odd-shaped, whereas planets are always spherical (or slightly oblate due to their spin). The reason obviously is that for sufficiently large masses/sizes the gravitational force overwhelms any resistance of even rocky materials to maintain a shape other than a sphere (which minimises the gravitational energy).

The characteristic scale at which this transition occurs depends on the material and is different for ice and rock as mentioned in this answer (to a related question) which also links this useful Wikipedia article. I wonder whether this limit can be estimated from simple physics (involving material constants for ice/rock).


texts:
My task is to calculate current declination and right ascension for some artificial Earth satellite. For example ISS.

I am using latest TLE data for extracting keplerian orbital elements and then calculate current cartesian orbital state vectors in ECI frame.
From the position vector I directly calculate declination and right ascension, using cartesian to spherical coordinates conversion.

Right ascension gets calculated correctly but I am having problems getting the same result for declination as the one displayed by satellite trackers.

Speaking of satellite trackers, I don't understand how is their declination value even correct.

Declination is an angle measured north or south of the celestial equator.

Celestial equator of course is on the same plane as the equator of Earth.

Then how is it possible that, when a satellite (ISS in this example) is almost directly above Earth's equator, satellite trackers calculate declination to be around -29° ?

Shouldn't the declination angle be zero?
My calculations for this "ISS above equator" example did give declination to be close to 0°.

Who is doing the calculations wrong and why?


texts: What was the definition of a planet before August 24, 2006?

texts:
In 2006, the IAU produced a definition of what it is to be a planet. This definition famously excludes Pluto, to the disarray of this small body's fans.
Before this decision, what was the definition of a planet?


texts: Do all planets rotate in the same direction in relation to each other?

texts:
In other words, does everything spin counterclockwise according to their axes (in relation to each other)? I suppose my question goes for stars, moons and other objects too.

But then again, all you have to do is look at any rotation upside down and it goes clockwise. So depending how you're looking at it, one could say everything rotates in the same direction whether they do or not. It's all a matter of perspective, right? And in space, there is no "right side up". Maybe my question is moot now that I think about it.

If we re-drew all of Earth's maps and globes upside down, New York or Sydney would be on the "west coast" and the Sun would rise in the "west" and set in the "east". Is it just by luck or maybe by the prominent early mapmakers, or that the majority of human population lives in the Northern Hemisphere, that it's given "dominance" on "top"? (I've heard that theory before)

Ugh. This is all so confusing.


texts: What natural mechanisms could lead to the unlikely case of the same rogue asteroid or planet passing through our solar system twice?

texts:
I find he following answer to the question What's the soonest Oumuamua could return? unsatisfying and oversimplifying and the last five words "Therefore it will never return." unsupported at a minimum.

  Oumuamua as an object is remarkable, because it has a positive net energy, which means it is not bound to the gravitational well of our sun. Therefore it will never return.

So I'd like to ask: *What natural mechanisms could lead to the unlikely case of a rogue asteroid or planet passing through our solar system twice?"

Stick to accepted scientific facts and theories
Let's give an ample time limit of 10 billion years between visits.
For the purposes of this question, the solar system includes the Kuiper belt (50AU) but not the Oort cloud
Don't include intervention by artificial means or intervention (divine or otherwise)


texts: Why can't we detect water on Mars in the same way as in exoplanets?

texts: Often I see news articles claiming astronomers found exoplanets with water. If we are capable of detecting water on such distant objects, why don't we employ similar methods to find water on Mars or Titan moon?


texts: How did Jocelyn Bell Burnell discover the periodicity of CP 1919?

texts:
According to Wikipedia,
in 1967, a radio signal was detected using the Interplanetary Scintillation Array of the Mullard Radio Astronomy Observatory in Cambridge, UK, by Jocelyn Bell Burnell. The signal had a 1.337302088331-second period and 0.04-second pulsewidth.[4] It originated at celestial coordinates  19h 19m right ascension, +21° declination. It was detected by individual observation of miles of graphical data traces. Due to its almost perfect regularity, it was at first assumed to be spurious noise, but this hypothesis was promptly discarded. The discoverers jokingly named it little green men 1 (LGM-1).
Also from Wikipedia,

My question is the signal was just measured from the chart directly?


texts: How can cosmological bubble universes "collide" with each other?

texts:
In Eternal Inflation Theory, there exists the idea of multiple "bubble" Universes that grow and according to this theory, can "collide" with each other. The paper suggests that you can search for evidence that our Universe collided with other "bubble" Universes, by looking for an imprint on the CMB radiation.

However, the concept of a Universe existing as a bubble that moves relative to other universes makes no sense:

How can you have a space "outside" of ours that allows the Universes to
move relative to each other?
In what dimension are the other "Universes" moving relative to ours?
If time started in our Universe at the Big Bang, then in what time dimension (outside of ours) do collisions between Universes occur?

PBS Spacetime looks at this idea here: https://www.youtube.com/watch?v=lrS0rxX-UhA

Edit: A winning answer will answer all three points above.


texts: I know that the radius of the observable universe is about 46.6 billion light years and is expanding at every instant of time due to dark energy . So is it possible that we our entire universe could be inside a black hole and that the edge of our observable universe represents the event horizon which in this case is expanding faster than the speed of light due to matter being constantly accreted inside it . Where that black hole is a part of another universe and that universe of another black hole and the pattern goes on . Is this possible ?


texts: Can Mercury's core "boil over" if it gets to close to the Sun?

texts: I was reading about how Mercury's core makes up most of the volume of the planet. I'm guessing this is both because of the small size and the distance to the Sun. If Mercury got too hot could it completely "liquidify"? Would it join the Sun and completely merge with it, gradually crack apart and spread, or something else entirely? Would it cause other problems in terms of life on Earth?


texts: Hypothetical scenario, two nearby black holes

texts:
I've always wondered what would happen if a hypothetical scenario, two black holes meet, with similar proportions:

Maybe one swallow the other one?

Or would make a bigger one between them.


texts: How does solar activity (e.g. flares, coronal mass ejections) change over the life cycle of a Sun-like star?

texts: As a Sun-like star is born from a collapsed nebula and generates energy by nuclear fusion some of that energy is converted to magnetic fields which fuel solar activity. How does that activity evolve as the star itself evolves?


texts: Is the Tombaugh Regio on Pluto visible from Earth?

texts: Is the Tombaugh Regio on Pluto discernably visible from Earth ground based or Earth orbiting telescopes? And more generally, can surface features seen in Hubble images be matched with images from New Horizons? I can imagine a match from the image compilation I attach, with TR being the bright triangle, but is that intuition valid?


texts: Why are there so few supernovae in M31 (Andromeda)?

texts:
Estimates of the supernova rate in the Milky Way put it at a few per century, but very few of these are visible from Earth (at least with the naked eye) because of intervening gas and dust clouds). On the other hand the Andromeda galaxy (M31) is sufficiently face on that we should see at least most supernovae there, and is several times more massive than the Milky Way, so one would naively expect to see at least one every few decades. Nevertheless, we have seen only one supernova in M31 in the last 150 years (SN1885a).

Are these accepted (or even seriously proposed) explanations for this?


texts:
Recently the Kepler telescope in its study of white dwarfs detected the first planetary object transiting a white dwarf in the data from the K2 mission. It was consistent with earlier theories' prediction that a planetary object orbiting a white dwarf would slowly disintegrate. Why would a planetary object that is orbiting a white dwarf disintegrate? I read it here.

The original paper published in Nature (preprint): A Disintegrating Minor Planet Transiting a White Dwarf


texts:
The sizes of the stars in the picture below have more to do with their brigtness than the actual size of the star.  I'm looking for a formula that relates the viewing size of a star with its brightness.

This is what I've worked on so far:

From Wikipedia on Apparent Magnitude, History

The viewing brightness can be related to the apparent magnitude by

$$B=0.4^A$$

Before I sit down and count pixels and try to do a curve fit to find the  the relationship, I'm guessing this work has been done before.  I wonder if anyone has any leads that would get me to a good equation that I can use to generate the view sizes of stars.

Edit:

My main goal is to make a reasonable 'cartoon' of the various constellations for the astronomy classes I'm teaching.  My rough work so far looks like this.  (I'm fairly certain the vertical scale is off.)


texts: Astrophysical particle spectroscopy; narrow-line particle sources (charged or uncharged) for things other than photons? Have any been detected?

texts:
Is Cosmic Ray Astronomy a thing?
Is there an equivalent of the red shift effect for cosmic rays?

have got me thinking and under this answer I've commented:

That's a good point; there aren't as many energy peak-producing process for particles as there are for photons, especially at high energy. For low energies there's alpha decay and internal conversion

That makes me wonder:
Question: Astrophysical particle spectroscopy; are there narrow-line particle sources (charged or uncharged) for things other than photons? Have any been detected? Have any instruments been built anticipating narrow peaks for things other than photons?


texts: What is the organic material in Saturn's rings?

texts:
The phys.org articleSurprising chemical complexity of Saturn's rings changing planet's upper atmosphere about the chemical complexity of Saturn's rings, quotes a member of the INMS (Thomas Cravens) as saying,

  What the paper is describing is the environment in the gap between the inner ring and upper atmosphere, and some of the things found were expected, such as water," Cravens said. "What was a surprise was the mass spectrometer saw methane—no one expected that. Also, it saw some carbon dioxide, which was unexpected. The rings were thought to be entirely water. But the innermost rings are fairly contaminated, as it turns out, with organic material caught up in ice. (italics and boldface are mine).

What is the nature of this organic material?

Does he simply mean a few carbon-based molecules like methane, propane, and butane, or maybe only methane,  or is it a much more interesting cocktail of chemicals than that?

It also links to Chemical interactions between Saturn’s atmosphere and its rings published in Science, but the article is paywalled.


texts: How could eclipse happen on the same day in a different calendar? (45 AD)

texts:
According to the eclipse calculator I use (Alcyone), a partial eclipse 0.314 magnitude occurred August 1, 45 A.D. in Rome starting at 8:35am local time.

This would seem to correspond to an eclipse which was predicted to fall on the birthday of the emperor Claudius in the consulship of Marcus Vinicius and Statilius Corvinus (45 AD), as described in Dio Cassius Book 6, Section 26. The birthday of Claudius was Kalends Augustus (the first of August).

If this is the case, it would seem that there is an exact correspondence between the Gregorian and Julian calendar going back to 45 AD. However, I would expect that small differences in leap years and other irregularities would make this exact match on the same day unlikely.

Another example is the eclipse of 1140 at London which the Saxon Chronicle says occurred on 13 Kalends April (March 20th) and Alcyone says occurred March 20, again a perfect match. Yet another example is the eclipse of 809 which passed over the Faroe islands and the Saxon Chronicle says occurred 17 Kalends Augustus (16 July) on the second day of the week. Now, of course, 16 July 809 is a Thursday in the Gregorian calendar, not the second day of the week, so something would seem to be fishy somewhere.

Is Alcyone/NASA making some kind of adjustment so that these pre-Gregorian dates match up?


texts: Is the process responsible for Triton's nearly perfect circular orbit going to happen in my fictional world?

texts:
First of all, I'd like to point out that I'm a worldbuilder and I like my worlds to be as physically possible as... possible.

I am in the process of building a world with a habitable moon orbiting around a gas giant in a similar fashion to Triton's orbits Neptune. I had originally intended it to be highly eccentric as well, but then I found out that Triton has a nearly perfect circular orbit. I looked into it a little and found something about tidal circularization.

What I'm not sure about is if my imaginary moon would undergo the process as well. Also, if it didn't, I'm not sure if it's even possible for life (and if, intelligent life,) to develop on a moon with such eccentric orbit.

Is it possible to explain what the parameters are that determine how quickly tidal circularization happen for systems similar to the Triton-Neptune system? How would the conditions need to be different for the system to remain in an elliptical orbit and avoid circularization?


texts: I have quasar spectrum which includes emission and absorption lines. I need to find absorption lines in between 2 wavelength. How can I do it with coding?


texts:
I'm setting up an observatory and I'm wondering what user-facing software would be best to use in terms of hardware control. There is a German mount, dome, focuser, filter wheel, various switches etc. All devices have / will have ASCOM drivers. For simplicity I'm setting up the operator PC to run Windows, but that is not set in stone (yet).

I have tried MaxIm DL and found it a pleasant surprise with all features it has available (e.g. dome slaving or autofocus).

What are some other software packages that I should try before deciding on (i.e. buying) one?


texts: High quality, detailed images of planets on web

texts:
I have been looking for images of the planets and have found pretty decent ones of Earth and Venus from NASA.

That detail is opposed to this one of Uranus:

And these of Mars, (and this and this)

I am disappointed in the quality of the public domain images from NASA I have found for these planets:

Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Earth's Moon

I am happy with the ones found for:

Mercury
Venus
Earth

They seem to be high quality in terms of detail and pixel size (in the MBs).

Wondering if one could explain how I go about finding these public domain planet images from NASA (or elsewhere, but I imagine they are all from the Hubble telescope, I don't know too much) that are high quality in terms of detail and pixels/size. If nothing else it would be helpful to simply link to them if there is no pattern to where they are located.

If there is a pattern to where the high quality images are located, I would be interested too to find ones of comets/asteroids/stars. I have found many high quality images of galaxies which is neat, but not of the planets yet.


texts: How long for the sun to traverse its diameter at the pole?

texts:
As the sun rises once per year at the north/south pole, the time taken to rise (and set) must also take significantly more time to complete to rise (and set).

How long does this take at either pole?

Interested to know if it is the same percentage of a day as anywhere else on earth (the day at the pole being several months long).


texts: Could liquid water have existed in open space 15 million years after the Big Bang?

texts:
Around 15 million years after the Big Bang, the ambient temperatures was about $24^\circ {\rm C}$, which is in a range where water could be liquid. Could liquid blobs of water be existent then?
PS: I am not talking about water on the surface of any solid planet.


texts: Why is the metallicity of elliptical galaxies low?

texts:
As far as I understand, metallicities of old stars are usually low and those of new stars are usually high.

If elliptical galaxies form after merger of galaxies, then surely that means the elliptical galaxy passed through a stage where it was an irregular galaxy. Now, irregular galaxies are said to be sites of rapid, new star formation.

Why then do we say that elliptical galaxies have low metallicity, if the star formation went on for a long time there?

[New to astronomy, so a detailed explanation would help]


texts: How to get the longitude/latitude from solar zenith/azimuth?

texts:
How do I get the longitude/latitude from zenith/azimuth?

Or how can I position my sun in a 3d application?

code extracted from here

public static AzimuthZenithAngle getSolarAzimuthZenithFromLatLon(float lat, float lon) {
   GregorianCalendar time = new GregorianCalendar(TimeZone.getDefault());
   return PSA.calculateSolarPosition(time, lat, lon);
}

public static Vector3 getSolarPosition(float radius, AzimuthZenithAngle az) {
    double a = az.getAzimuth();
    double z = az.getZenithAngle();

    float X  = (float) (radius*Math.sin(z)*Math.cos(a));
    float Y  = (float) (radius*Math.sin(z)*Math.sin(a));
    float Z  = (float) (radius*Math.cos(z));

    return new Vector3(X, Y, Z);
}

this is my code, but it's not working very well.


texts: What is the format of the data from the JPL's HORIZONS system?

texts:
I'm looking for accurate positions and velocities of the planets in the solar system over several decades. I want to simulate their trajectories using Newtonian laws of motion, and compare the precession of Mercury's perihelion with observed data. I used the JPL's HORIZONS system but I can't manage to find these positions and velocities in the output data.

Could you explain how the data is formatted? (The HORIZONS web-interface Tutorial didn't help.)


texts: What percentage of gas giant exoplanets are in the habitable zone?

texts: Approximately what percent of exoplanet gas giants discovered so far are in the habitable zones of their stars? I hear that most of them are "Hot Jupiters", orbiting so close to their stars that the heat causes them to swell up like a hot air balloon.


texts: Why is cosmological event horizon closer to us than the particle horizon?

texts: From what I learned, cosmological event horizon is the horizon that moves away from us at/more than the speed of light. And particle horizon is the horizon that particles inside the horizon can have a causal connection with us. It is the maximum distance from which particles could have traveled to the observer in the age of the universe. This simply indicates a contradiction to me. How is it possible that the cosmological event horizon is closer?


texts: 3 years later, how strong is the evidence for planet 9?

texts:
I came across post in quora where somebody said that "most astronomers no longer think Planet 9 exists" and being somewhat shocked by that, I did a google search and found a fairly recent article that claims exactly that.

It can take several years to prove a negative within pretty good certainty, as opposed to a discovery which can be checked pretty quickly.   Because I don't want to wait several years, I wanted to ask if the general consensus is that Planet 9 probably doesn't exist?  Or is there still a respectable amount of belief among astronomers in the original evidence and any new evidence that's followed?

If it ends up not existing, I'll be sorry to hear it.   It was a fun story when it lasted.


texts: Which telescope would be better with a DSLR?

texts:
I'm interested in astrophotography and since I haven't left my home in six weeks, my interest in photographing the skies has been growing. I'm on a tight budget and I've found these two options:

A Meade Infinity 70mm (f/10), a cheap refractor.
The Omegon Maksutov MightyMak 80mm, only for 20€ more.

The Meade has a T2 mount, the Omegon uses 1.25" but comes with a T2 adapter; meaning in both cases I'd need the T2 to Canon adapter.

I would also like to be able to use it for terrestrial photography once the lockdown is over, and that's a big factor in my case, even if the results are not excellent. I know that for astro the mount is extremely important, but I'm not thinking about deep sky, mostly planets and the Moon and, of course, terrestrial.

In most forums they always talk about expensive options, but these cheap ones are not often discussed for obvious reason.


texts: How do scientists know if an Earth-like planet is really Earth-like?

texts: I'm not sure how scientists determine that a planet is really an Earth-like planet. How do they do it?


texts: How is the entropy of a blackhole stored at its horizon?

texts: I read that a blackhole has an entropy that is given by the area of its event horizon (measured in Planck units). Does that mean that this information is actually physically stored there? If yes, how can that be since there may be nothing there at all (no electrons for example that could carry this information in terms of spin up or spin down)? Is it perhaps stored in the vacuum somehow ?


texts: Number of planets in a habitable zone with large moons

texts: Excluding gas giants, roughly what percentage of planets in the habitable zones of their stars would have large moons like Earth does, as opposed to Mars and its small moons/captured asteroids?


texts: Are the Jupiter-Saturn conjunctions and winter solstice related?

texts:
There is conjunction of Jupiter and Saturn on Dec 21. And there is another celestial event on that day the winter solstice.
So I'm just wondering that are these two events are related or not?


texts: Why is dark energy preferred to the cosmological constant?

texts:
Dark energy and the cosmological constant are often identified, but Peebles and Ratra explain that

  "Einstein did not consider the cosmological constant to be part of the stress-energy term... a new constant of nature,
  $\Lambda$, appears in the addition to Einstein’s original field equation. One can equally well put Einstein’s new term on the right hand side of the equation, and count [...] as part of the source term in the stress-energy tensor. The distinction becomes interesting when $\rho_\Lambda$ takes part in the dynamics, and the field equation is properly written with $\rho_\Lambda$, or its generalization, as part of the stress-energy tensor."

In other words, Einstein treated $\Lambda$ term as a modification of GR with a "new constant of nature", while dark energy may potentially manifest itself in some other ways. Later they write about dark energy:

  "The idea that the universe contains close to homogeneous dark energy that approximates a time-variable cosmological “constant” arose in particle physics, ...in cosmology..., and on both sides by the thought that $\Lambda$ might be very small now because it has been rolling toward zero for a very long time. The idea that the dark energy is decaying by emission of matter or radiation is now strongly constrained by the condition that the decay energy must not significantly disturb the spectrum of the 3K cosmic microwave background radiation."

So if I understand this correctly the idea of a dynamical effect came from suspecting that $\Lambda$ may be time dependent, but so far that was not detected. And since "one can equally well put Einstein’s new term" on either side of the equation mathematically both interpretations are equivalent. Still, the dynamical interpretation seems to be overwhelmingly preferred.

  Are there empirical reasons for conjecturing dark energy, some spatial non-uniformity in expansion perhaps? If not, what are the theoretical reasons for preferring it to the cosmological constant of modified GR?


texts: Could the Earth be used to cast an arago/poisson spot on something

texts:
There's a potential design for a space-based telescope that uses a large opaque circular disk in place of a transparent lens to focus light for analysis. It uses the light that bends around the disk that focuses on the Poisson or Arago spot, but requires a circular object. It could theoretically produce much clearer images (1000x) than telescopes we have today, albeit with a narrower field of vision.

I understand that the Earth is not perfectly round, but I believe I've also heard it's very "smooth" despite mountains and such, so I wonder if instead of a spot (very small circle) on some point behind it, it might cast an oval or other oblong shape of some size that, using lenses or computer reconstruction, might produce an image that is usable, and presumably even higher resolution (millions, billions, trillions, or even more times as detailed?).

If that's not possible, what are the issues? Does the atmosphere get in the way? If we could imagine a sufficiently circular disk roughly the size of Earth, about how far would the image processing receiver have to be from the disk opposite the studied object in order to capture the convergence of bent light? Would the gravity of the Earth affect the distance at all, due to gravitational lensing?


texts: How Can I interpret this particular light curve without reading data on the side

texts:
There is only one dip in flux recorded. How can I interpret this particular light curve and find planetary radius
https://exo.mast.stsci.edu/exomast_planet.html?planet=55Cnce


texts:
I don't know how common it is for a system to have two stars (or perhaps even more) but how do they arise? Is that due to the stellar accretion disc, or the composition of the stellar nebula?
Or are most of them created by a collision of stars?


texts: How far away was the binary black hole that produced GW170729?

texts:
GW170729 was gravitational wave signal's detected by the Virgo and LIGO detectors. I read the following in a news report:

  Known as GW170729, in accordance with the date of its discovery (in US
  format), the gravitational wave that astronomers can listen in to was
  caused by the formation of a black hole more than 80 times as massive
  as our sun and about nine billion light years away. (Source)

I am specifically interested in the claim that it comes from 9-billion light years away. I have been unable to find a good source to back this claim up.

How accurate is this claim? Please provide a source for your answer and where applicable, a simple explanation of how the data backs this up. More information about this event can be found here.


texts: If the Moon has a magnetic field and can possibly receive solar wind, then does it have an aurora? If Earth has an aurora, and Saturn has an aurora, then could the Moon possibly have an aurora?


texts: Jupiter, Saturn, Uranus, Neptune system GM

texts:
I'm modelling the solar system. I'm using data from Nasa.
To do this I'm using the barycenter of the planets Jupiter, Saturn, Uranus, Neptune. However the website fails to give the Gravitational Parameter $\mu$ for these barycenters, even though it does do so for Pluto's barycenter.
To make matters more confusing, the $\mu$'s given at Wikipedia are slightly different, though I assume these are just out of date.

So I'd like to know if there's an as up to date resource as possible available for these parameters. Paywalls should be OK as long as they are of major publications.
$$\frac{\mathrm{AU}^3}{\mathrm{M}_\odot\cdot\mathrm{day}^2}$$
-form also good.


texts:
How can a national participation moment (period) be planned?
Impractical, but as an idea useful for imagining a rationale for this question:

Starting in NYC and moving across the country to San Francisco, at NYC sunset today, June 02,2020, someone in NYC "Bends a Knee" in support of an idea the nation is engaged in. As the sunsets across the nation, person(s) along an imaginary strait corridor stretching from NYC to SF,also takes a knee, and this cascades across the nation as the sunsets. When SF folks have spent 3 minutes on that knee, the entire country, NYC to SF, moves to their other knee and remains for 1 minute, then everyone rises, and heads back inside, or home.

Perhaps there are some extremely dedicated folks along that straight line in NYC that could manage to be be on one knee for the duration, rising only when the folks in SF do too.

Imagine that.  I would like to try and pull it off, very soon but haven't a clue how to make a time line for it on social media, essentially twitter.

If this of interest to someone with greater standing on this site, a tag #SunsetTake1Knee might help this idea alone if it were to come to be. I just joined and have no rights for tagging.


texts: How to calculate the time of Ekadashi days?

texts:
Is there an official reference algorithm for calculating the Ekadashi days?

I found the textual description of Ekadashi calculation on wikipedia but can't really make sense of it in order to write the code.

I already have the (java) code for calculating the new/full moon phases, and originally assumed that Ekadashi would be as easy as appending 11 days to the time of new and full moon but apparently this approach is not providing correct results.

Update:

Based on James's answer the answer is 11 lunar days and 26 lunar days after new moon (not gregorian days).

The required conditional adjustments are described in chapter 1.4 of Vaisnava Events Calculation which don't match the ones described in the wikipedia article.


texts: Can a bullet be fired on the moon and sent it into orbit?

texts:
I know you need oxygen for the ignition, but presumably, if the bullet is impervious (or water-tight) and if there is a little air enclosed in the bullet case, wouldn't that be sufficient to fire it ?

And, giving the speed, would that be enough/too much to send it into orbit around the moon ? Would you need to fire it at a certain angle ?(which will presumably make it an elliptic orbit)


texts: Might the universe be really small yet seem infinite from spherical reflections?

texts:
if a single source of light quite small, was placed somewhere inside a completely reflective sphere, quite large, what would the reflections look like from the light source's point of view within that sphere?

:)

extra-credit: what would it look like over the course of time with movement starting from the moment the light turned on?

Found this question has been asked over in MathOverflow, sum clue?


texts: Explanation about the resonance, mean motion resonance and libration

texts:
I am studying for a project the dynamics of multi-planetary system in resonance. I have two questions about this domain.

In many articles they speak about 'mean motion resonance', what is the difference with a classic orbital resonance?
How can we define that two exoplanets are in resonance? Scientific articles are using the ratio of the orbital periods being related by an integer ratio, but in a book (exoplanet handbook-Michael Perryman) I saw "mean motion resonance: definition is via the librating or oscillating behaviour of
some linear combination of resonant arguments". What is the difference between this or the ratio of periods to define the resonance?

Also, if you have any recommandation of books/articles about this topic, I would be glad to hear it.

Thank you!


texts:
In 1939, Robert Oppenheimer and other concluded that a certain neutron stars could collapse to black holes and no known law of physics would intervene.  As far as I can tell, there is no observational data that shows that an event horizon of a black hole exists.

Assuming that a black hole with an event horizon exists, then there are all kinds of mathematics that explain what is observed externally, approaches the event horizon.  It can be explained what an infalling observer sees as they approach the event horizon.  However, what happens when an infalling observer passes the event horizon, the result is a mathematical absurdity, in that the observer would observe the universe outside the event horizon as beyond "the end of time."

It seems that passing through an event horizon is as mathematically absurd as an object with non-zero mass reaching and surpassing the speed of light, as it would require more than infinite amount of energy in the latter, or more than an infinite amount of time in former.

So, can an event horizon even "exist" in the first place, if "existing" means within the bounds of any moment after the big bang and before an infinite amount of time from any point within the universe.

It seems that the existence of black holes seems to be well accepted, however faster than than light travel does not under the current understanding of the physics of relativity.


texts: Solar wind may contain some hydrogen and oxygen atoms which is the composition of water, some of them may move to the poles of earth, my question is, is solar wind also a source of water in earth?


texts:
Does anyone have any idea were I can get a dataset of stars in the milky way galaxy, their mass and their planets and distance of planets from their star?

I have tried various catalogs of NASA. I need something specific. I am planning to run some data analytics on them. So it needs to be in a format I can use preferably excel or data.


texts: How to calculate planetary aspects at a stationary point?

texts:
I am using Jean Meeus book Astronomical Algorithms and the C software that comes with the book to calculate planetary data. I am comparing two planet's longitude to calculate the time when they are a fixed amount of longitude apart such as 45, 60, 90 degrees apart. These are the standard planetary aspects used in astrology.  I can not calculate the correct time when one or both of the planets is approaching its stationary retrograde motion point. At this point the planets velocity approaches zero and the longitude does not have the decimal point accuracy to make a comparison. The time will be off by anywhere from a few minutes to eleven hours when compared to a commercial ephemeris.

Q.) Does anyone know a different method other than simple longitude comparison that can calculate the correct planetary aspect time when planets are near their retrograde motion stationary point?

-- Edit is Below --

Here is an example of my problem. My calculations below are made with Astronomical Algorithms software at 1 minute increments for August 1, 2016 at midnight. The calculations show Venus longitude which is moving Direct Motion and Uranus which is moving Retrograde Motion. The American Ephemeris at midnight lists the Venus 120(Trine) Uranus as occurring at 4:10 AM. The Astronomical Algorithms calculations below identify Venus 120 Uranus at 3:56AM which is 16 minutes early. Every time a slow outer planet like Uranus is near a station point and moving very slow my aspect time is always off.

Q) Is there a different way to calculate the aspect rather than longitude comparison? Maybe use some king of time correction function?

Geocentric Longitude

Venus Direct Motion

Uranus Retrograde Motion

Date ___ Time _______ Venus _______ Uranus

08/01/16 03:53 AM   144.5040255637  24.5061221269

08/01/16 03:54 AM   144.5048790144  24.5061207491

08/01/16 03:55 AM   144.5057324652  24.5061193708

08/01/16 03:56 AM 144.506585916   24.5061179922  Venus 120 Uranus

08/01/16 03:57 AM   144.5074393668  24.5061166132

08/01/16 03:58 AM   144.5082928175  24.5061152338

08/01/16 03:59 AM   144.5091462683  24.506113854

08/01/16 04:00 AM   144.509999719   24.5061124738

08/01/16 04:01 AM   144.5108531697  24.5061110932

08/01/16 04:02 AM   144.511706621   24.5061097123

08/01/16 04:03 AM   144.5125600718  24.5061083309

08/01/16 04:04 AM   144.5134135225  24.5061069491

08/01/16 04:05 AM   144.5142669732  24.5061055669

08/01/16 04:06 AM   144.5151204239  24.5061041844

08/01/16 04:07 AM   144.5159738746  24.5061028014

08/01/16 04:08 AM   144.5168273253  24.5061014181

08/01/16 04:09 AM   144.5176807759  24.5061000343

08/01/16 04:10 AM 144.5185342266  24.5060986502 American Ephemeris Venus 120 Uranus

08/01/16 04:11 AM   144.5193876773  24.5060972657


texts: How many galaxies disappear beyond the Hubble Bubble horizon every year now?

texts:
The accelerating expansion of space means that the space between us and far away galaxies expands faster than light can travel through space. There is a horizon of possible observation beyond which galaxies disappear from us.

At what rate do galaxies, or some other measurement of mass/energy, disappear beyond that horizon at present rate? Will future telescopes actually see galaxies red shift themselves into oblivion? I imagine there being issues about the definitions of "when" and "where" in this context. Answers should try to address how they deal with that.

Since the radius of the observable universe expands at the speed of light, my intuition tells me that there should've been a point in time when the expansion of space balanced the arrival of light of more distant parts of space. if so, when did that happen?


texts: Why did the Event Horizon Telescope take so long to take a photo of a black hole?

texts: The Event Horizon Telescope is in the news now because they are about to release the first ever picture of a black hole. They have been working on this since 2006. Why did it take 13 years to produce the first picture? What was the critical mass that was recently hit that made the picture now complete?


texts: The star Polaris is commonly known as the Northern Star because it always points North. This being said is there a South star that always points south?


texts: The relation between the light of full moon and the distance between the sun and the earth

texts:
If we want the full moon to be 2 times more bright,how we should change the distance between the sun and the earth?

Thanks for your help.


texts: What are the key coordinate systems used in Astronomy, and where are they used?

texts:
Wikipedia describes the five following celestial coordinate systems each with rectangular/spherical variants:

horizontal
equatorial
ecliptic
galactic
supergalactic

What are the practical preferred uses of each, in relation to the others. If you were navigating within the solar system, which system would you use, or has been used by existing craft?

For example wikipedia describes the ecliptic system as "still useful for computing the apparent motions of the [celestial bodies]", which seems pretty bloody obsolete. Even though it seems to me as the most intuitively useful system.


texts: Can angular information be known more precisely than the diffraction limit?

texts:
The diffraction limit deals with the ability to determine if two things are separate.  I am interested in the ability to find the the centroid of a single object.

Imagine a star with no near neighbors would it be possible to determine the centroid of that star at a resolution higher than the diffraction limit?

The star is essentially a point source and I wonder if looking at the edges of the airy disk would allow a centroid to be determined if I had a magical camera with infinite pixels.

It seems to me that I should be able to see the disk and then calculate a centroid that is smaller than the airy disk.

Thanks for any help you can provide =)


texts: How was calculating the celestial latitude using an astrolable of any help to sailors?

texts: I'm sorry if this sounds like a silly question but I don't understand how measuring the location of a star would help in sea navigation.


texts:
It's generally easier to be certain an object is a star than a black hole. HDE 226868 is one of our site moderators and a blue supergiant star. The latter has a 5.6 day companion that supplied a magazine subscription to Kip Thorne:

  According to the updated tenth-anniversary edition of A Brief History of Time, Hawking has conceded the bet due to subsequent observational data in favor of black holes. In his own book, Black Holes and Time Warps, Thorne reports that Hawking conceded the bet by breaking into Thorne's office while he was in Russia, finding the framed bet, and signing it. (While Hawking referred to the bet as taking place in 1975, the written bet itself (in Thorne's handwriting, with his and Hawking's signatures) bears additional witness signatures under a legend stating "Witnessed this tenth day of December 1974". This date was confirmed by Kip Thorne on the January 10, 2018 episode of Nova on PBS.)

Question: What observational data convinced Stephen Hawking that Cygnus X-1 was indeed a black hole and caused him to break into Kip Thorne's office?


texts:
I am new to this site so if the question doesn't fit in this forum feel free to remove it.
I have read here https://en.wikipedia.org/wiki/Sunburn of all the causes of sunburn and I was mostly aware of all of them. I will list them here:

($1$) The time of day. In most locations, the sun's rays are strongest between approximately 10am and 4pm daylight saving time.
($2$) Cloud cover. UV is partially blocked by clouds; but even on an overcast day, a significant percentage of the sun's damaging UV radiation can pass through clouds.
($3$) Proximity to reflective surfaces, such as water, sand, concrete, snow, and ice. All of these reflect the sun's rays and can cause sunburns.
($4$) The season of the year. The position of the sun in late spring and early summer can cause a more-severe sunburn.
($5$) Altitude. At a higher altitude it is easier to become burnt, because there is less of the earth's atmosphere to block the sunlight. UV exposure increases about 4% for every 1000 ft (305 m) gain in elevation.
($6$) Proximity to the equator (latitude). Between the polar and tropical regions, the closer to the equator, the more direct sunlight passes through the atmosphere over the course of a year. For example, the southern United States gets fifty percent more sunlight than the northern United States.

My question is, can we compose ($1$), ($4$) and ($6$) into one statement: "Height of sun over the horizon"?
Wikipedia seems to hint that we can since it says:

Regardless of one's latitude (assuming no other variables), equal shadow lengths mean equal amounts of UV radiation.

However, the statement is kind of vague and I would like confirmation.
Another doubt I have regarding this is experience. I live in Europe where solar noon is at 1pm during summer and the day is longest on June 21 (or 22, whatever). But, people seem to get way more sunburn in, say, August 21 than in April 21, and also more at 4pm than at 10am (regardless of the month). It is also hotter in August than in April and at 4pm than at 10am. So it might be that people simply spend more time out because it's hotter and therefore get more sunburn, but it also might be that the temperature affects our skin somehow and we do get more sunburn when it's hotter.
So, taking clouds, reflective surfaces and altitude out of equation, is it true that sunburn is caused just by the height of the sun above horizon? If I spend 5 minutes at some place on a sunny day at noon in April, will I get the same amount of sunburn as I would at 3pm in June? (given that the sun is at the same height over the horizon at that times, and that 5 minute span is short enough to approximate that sun stays "fixed" in the sky)
My intuition says it should be true that height over horizon is the only parameter that should matter, but my experience shows different. Moreover, if I am correct, is there some graph/table showing how much intensity (or some other unit?) of sunburn I get with respect to height of sun? (say at zero altitude on a sunny day without reflective surfaces, or at some other, fixed, reference configuration).


texts: If light has no mass, why is it affected by gravity?

texts: Light doesn’t accelerate in a gravitational field, which things with mass would do, because light has a universally constant velocity. Why is that exception?


texts: Which star did Uranus occult in 1977 when the planet's rings were discovered?

texts: In 1977, Uranus occulted a star, and the starlight flickered 40 minutes before and after the actual occultation, revealing Uranus' rings.This is common knowledge to astronomers, but what I can't find out is which star Uranus occulted. Can someone give me the name of this star?


texts: How can astronomers determine the difference between "hydrostatic equilibrium" and "just happens to be spherical"?

texts:
This is relevant for the definition of a dwarf planet.

I presume the answer will be, well, if we can tell the mass of the body and guess the material.   I don't find this very satisfactory because (1) may be impossible and (2) will have large error.


texts: Interaction of stellar winds in binary star systems

texts:
Binary star systems are fairly common but I am unable to find any information on how the stars interact with one another on aspects other than gravitational affects.  Using Alpha Centauri as the candidate system, what possible effects could happen where their stellar winds reach one another?
Would a cloud of magnetized gas form? Would there be a highly radioactive area between them? Could it form a sort of micro-nebula between the stars?


texts: What is the upper and lower limit of temperatures found on stars?

texts: What are the most extreme temperatures (both hot and cold) stars have been detected  at?  Is there an upper and lower limit for the detected temperature of stars?